Reconfigurable intelligent surface (RIS), also known as intelligent reflecting surface, or large intelligent surface, has been recognized as a promising next-generation wireless communication technology [1]}, [2]}, [3]}. An RIS consists of a large number of software-controlled meta-atoms, each of which can independently impose the required phase shift on the incident electromagnetic waves. By carefully adjusting the phase shifts of all the meta-atoms, the radiated electromagnetic waves can be shaped to propagate towards desired directions, thereby significantly enhancing the communication quality. In wireless communications, RISs have many potential applications. For example, in RIS-enhanced cellular networks, RISs are deployed to establish favorable non-line-of-sight links between base stations (BSs) and users. Thus,
additional degrees of freedom are introduced by RISs for improving the system performance [4]}. Other applications include the beamforming design in simultaneous wireless information and power transfer networks [5]}, [6]}, unmanned aerial vehicle aided wireless networks [7]}, and internet of things networks [8]}.
